One alternative to the production of paper pulp by conventional kraft and sulfite chemical pulping technologies in solvent pulping. Most proposed solvent pulping processes, such as disclosed in U.S. Pat. Nos. 4,764,596 and 4,100,016, use alcohol as a solvent, particularly an ethanol and methanol mixture. The alcohol is introduced with wood chips into a batch digester, and after cooking the material is subjected to three different washes in the batch digester, the first wash with a slightly weakened cooking liquor (containing alcohol), the second wash with a weak cooking liquor, and the third wash with water. One of the proposed advantages of the solvent pulping technique is that lignin may be recovered from the "black liquor" produced from the process (a solution of lignin in a water miscible organic solvent such as a lower aliphatic alcohol). It is necessary, in order to make the system economical to recover as much of the alcohol as possible. Significant markets may also develop for the lignin, which may make solvent pulping economical and advantageous.
At the present time, there are no known large scale commercial installations in which solvent pulping is practiced. One of the most significant reasons for this is the inability to recover a substantial enough portion of the alcohol. If one utilizes a batch digester, with washing in the digester, as/described above, the alcohol consumption may be such as to make the procedure economically unattractive.
There are certain problems associated with proposed solvent pulping systems. One is the potential safety hazard as a result of solvent vapor, oxygen (i.e. an oxidative gas), and a condition--such as a spark--capable of producing an explosion, combining. In order to guard against this, when the operation of the batch digester is being arrested or terminated, any portions thereof where vapor can collect are purged with nitrogen, or a like substantially oxygen free gas.
It has been recognized for many years that the solvent pulping process could theoretically be improved if it were made continuous, such as the majority of commercial kraft and sulfite pulping systems. However the safety problems described above, plus the need for equipment to maintain sufficient pressures to accommodate solvent pulping (which pressures are much higher than for kraft pulping) made the realization of that ideal difficult to achieve. It was also recognized that the lack of recovery of a substantial portion of the alcohol as a result of washing was a major drawback, but techniques for significantly reducing the alcohol loss were not envisioned.
According to the present invention, it is possible to make the solvent pulping process continuous. Also, according to the present invention, it is possible to wash pulp produced by solvent pulping (either by a continuous process or batch process) so that the alcohol loss per ton of pulp is at an economical level (e.g. about ten gallons or less; an economically acceptable level).
In the design of equipment to make the solvent pulping process continuous, to the extent possible conventional Kamyr.RTM. vessels and equipment from kraft and sulfite chemical pulping processes are utilized. However it is necessary to provide additional equipment, reconfigure the equipment, and substitute components capable of handling higher pressure, in order for the system to work effectively.
In the production of washing equipment which can effectively wash the lignin from the pulp, and also wash the alcohol therefrom so that a substantial portion of the alcohol is effectively recovered, again conventional Kamyr.RTM. and Ahlstrom equipment is utilized to the maximum extent possible. However the equipment must be configured in a novel system, and various changes made thereto.
According to one aspect of the present invention, apparatus is provided for steaming comminuted cellulosic fibrous material chips for feeding from a high pressure feeder to a continuous digester, and a method for steaming such chips during solvent pulping thereof. The apparatus comprises: A chips bin, having a chips outlet at the bottom thereof. A first horizontally extending steaming vessel having a chips inlet and outlet, a steam inlet, and a gas vent. A second horizontally extending steaming vessel having a chips inlet and outlet, and a gas vent. A first low pressure feeder between the chips bin outlet and the first steaming vessel chips inlet. A second low pressure feeder between the second steaming vessel chips inlet and the first steaming vessel chips outlet, including a first conduit extending from the second low pressure feeder to the second steaming vessel chips inlet. A second conduit extends from the second steaming vessel chips outlet and is connected to the high pressure feeder inlet. And, means for introducing steam into the second steaming vessel through the first conduit so as to flow with chips from the second low pressure feeder into the second steaming vessel chips inlet.
The gas vent from the second steaming vessel extends upwardly therefrom on the discharge end, and the second conduit extends downwardly from the second steaming vessel generally opposite the gas vent. For safety, means are provided for introducing a substantially oxygen free purging gas into the second conduit to flow upwardly into the second steaming vessel during shutdown of the apparatus. Solvent recovery means is operatively connected to the steaming vessel gas vents.
In the method of steaming cellulosic fibrous material during solvent pulping thereof according to the invention, first and second steam zones are utilized. The method comprises the steps of continuously: (a) Adding steam to material in the first steaming zone while maintaining the pressure at about 10-20 psi. (b) Isolating the first steaming zone from the second steaming zone. (c) Maintaining the pressure in the second steaming zone at about 20-75 psi. (d) Purging the second steaming zone with steam by introducing steam into the material to flow concurrently with the material into and through the second steaming zone. (e) Venting gases, including vaporized solvent, from the fist and second steaming zones. (f) Discharging steamed material from the second steaming zone to the high pressure feeder. And, (g), adding solvent to the discharged material from the second seaming zone prior to its introduction into the high pressure feeder. The material is moved generally horizontally within the first and second steaming zones, and step (g) is practiced by adding ethanol as the solvent, preferably with about 10% methanol added thereto. The second steaming zone is purged with nitrogen or other substantially oxygen free gas when the practice of steps (a)-(g) is arrested or terminated.
According to another aspect of the present invention, the digester vessel itself is configured so as to minimize the risk of explosion and to maximize extraction of lignin containing liquid. In the solvent pulping process the ratio of liquid to cellulosic fibrous material is much higher than in kraft pulping, typically on the order of about 6-9 to 1, as opposed to a 4-5 to 1 ratio for kraft pulping. These goals are accomplished according to the invention by utilizing a vessel free of mechanical liquid/material separating devices at the top thereof. Heretofore, all single vessel continuous digesting systems have utilized a mechanical separator at the top, typically a screw rotating in a perforate cylinder. According to the invention, however, separation is accomplished utilizing a plurality of screens, and controlling the operation of the screens so that liquid is periodically withdrawn from one, then withdrawal is terminated, and then started again, etc., at all times at least some of the screens operating, and at all times at least some of the screens being dormant. The excess extraction is handled by adding screens to the digester, and--in a recirculating loop between the central extraction portion for the lignin containing liquor, in the top of the digester--removing a portion of the withdrawn liquid from the recirculating loop, sending that removed portion (which contains a substantial amount of lignin) to lignin recovery, and making up for the removed portion with fresh solvent cooking liquor, which is heated with the recirculated liquid and reintroduced into the digester via the central pipe bundle, exiting in the center of the chip column at an elevation slightly above or below the respective extraction screen. The recirculating loop screen and system comprises a withdrawal conduit having an isolation valve and a flow control valve, and a replacement liquid conduit having an isolation valve and flow control valve.
The method of solvent pulping to accomplish the objectives set forth above is practiced by the steps of continuously: (a) Steaming the material to remove the air therefrom. (b) Mixing the material with solvent pulping liquid to produce a mixture. (c) Feeding the mixture of material and solvent pulping liquid under pressure to the top of the vessel. (d) Separating some liquid from the material at the top of the vessel in a manner positively precluding the generation of electrical or mechanical sparks. (e) Returning the separated liquid from step (d) to step (b). (f) Withdrawing a liquor having a high concentration of dissolved lignin from a central portion of the vessel. And, (g) withdrawing produced pulp from the bottom of the vessel. Step (d) is practiced by the steps consisting essentially of providing a plurality of screens at the top of the vessel, withdrawing liquid through at least one screen while liquid is not being withdrawn through at least one other screen, and periodically switching which screens liquid is and is not being withdrawn through. During arresting or terminating the practice of steps (a)-(c), oxygen free gas is passed through the material countercurrent to the normal direction of flow of the material to prevent explosive vapor from collecting. Countercurrent diffusion washing of lignin from the pulp begins in the lower portion of the digester vessel where filtrate from the external washing stages is introduced and flows upward through the vessel counter to the flow of chips. The rate of flow of washing medium counter to the chip flow in the digester will be in the range of 1-4 tons of alcohol/water mixture per ton of dried pulp leaving the digester.
Washing of the pulp produced by solvent pulping--either by a batch or continuous process--is accomplished in a number of stages, at high pressures. The first stage is preferably a pressure diffuser, which is capable of operating at up to about 600 psi, and typically will operate at a pressure of at least about 350 psi, typically about 425-450 psi. In the pressure diffuser, the lignin is washed out of the pulp utilizing as the wash liquid a mixture of solvent and water, typically at least about 50% ethanol-methanol, and the rest water. The pulp from the pressure diffuser passes to storage vessel, and then to a first multi stage drum displacer washer, and then to a second multi stage drum displacer washer. Fresh water washes alcohol from the pulp in the second multi stage drum displacer washer, with the spent wash liquid therefrom used as the wash liquid in the first multi stage drum displacer washer, and the spent liquid from the first multi stage drum displacer washer used--with make-up alcohol--in the pressure diffuser. In all of these vessels it is necessary to purge any portions thereof where vapor may collect during normal operation and when operation is arrested or terminated, the purging taking place using nitrogen or a like substantially oxygen free gas.
The lignin rich spent wash liquid from the pressure diffuser, which also contains a large amount of alcohol, passes through a fiber filter. A first stream--which has been filtered so that it is substantially free of fibers--then passes to lignin and alcohol recovery, while a second stream--which still has fibers therein--is returned to the pulping system, to be introduced into the steamed chips as part of the solvent mixture slurrying the chips for the high pressure feeder, or into the bottom of the digester vessel to be used as wash medium in the countercurrent diffusion washing zone. Utilizing such a washing system it is possible to recover substantially all of the alcohol, that is all except for about ten gallons or less per ton of pulp produced.
It is the primary object of the present invention to provide a continuous solvent pulping method and apparatus, and/or to provide for effective washing of pulp produced by solvent pulping, so as to maximize alcohol recovery. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.